DRIFTS study of Fe promoter effect on Rh/Al2O3 catalyst for C2 oxygenates synthesis from syngas
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DRIFTS experiments such as CO adsorption, CO-TPSR and CO+H2 were designated to study the effect of Fe promoter on the key steps of C2 oxygenates formation from syngas. The CO adsorption results demonstrated that Fe weakened CO adsorption and especially the bridging adsorption. It was found in CO-TPSR experiments that the catalyst with lower Fe loading is more easily dissociated while the ones with higher Fe loading own stronger hydrogenation activity. Moreover, it was observed by CO+H2 experiments that Fe plays a role in stabilizing the lineally adsorbed CO species and decreasing the CO desorption rate. The catalytic performance results indicated that when Fe content is 4wt. %, the selectivity of total C2 oxygenates is the highest, which was in accordance with the DRIFTS results.
KeywordsDRIFTS Fe promoter Rh-based catalysts Syngas
Rhodium-based catalysts have been paid considerable attention due to its high selectivity to C2 oxygenates such as ethanol from syngas [1, 2, 3]. Meanwhile, it is well known that Fe is an efficient promoter of rhodium-based catalyst [4, 5]. For instance, a series of Rh-Fe/Al2O3 catalysts were prepared by Burch et al.  where they observed that Fe inhibited the formation of CH4 and increased the selectivity of C2 oxygenates as well. Chen et al.  found that the optimized Fe content was 4 wt % for obtaining the highest ethanol yield over Rh-Fe/Al2O3 catalysts. Yu et al.  reported that the optimized Fe loading over Rh-Mn–Li/SiO2 was 0.1 wt %. Furthermore, some possible mechanisms about the promotion of Fe.
Promoter had been discussed, although they were still controversial. Schunemann et al.  attributed the improved activity of rhodium-based catalyst to the highly dispersed Fe3+ (Fe2+) oxides in close contact with Rh particles, which favored the ethanol and ethyl acetate formation. Choi et al.  concluded that Fe increased the reaction energy barrier of methane formation, thereby increasing the ethanol selectivity.
Wang et al.  reported that the addition of the Fe promoter brought an increase in the interfacial region between Rh and Fe.
It is well accepted that mechanism of ethanol formation is determined by such steps such as CO adsorption, CO dissociation, CO hydrogenation and CO insertion . Therefore, it is helpful to understand the effect of Fe promoter on the investigated catalysts if we know how it affects these mechanism steps. In situ diffuse reflectance infrared transform spectroscopy (DRIFTS) is widely used to study the interaction of probe molecular with catalyst surface, which was found to be very sensitive to adsorption sites, CO coverage and surface orientation [13, 14, 15]. In CO hydrogenation reaction, CO adsorption behavior on the Rh-based catalysts was investigated and some meaningful conclusions had been reached. For example, Yu et al.  concluded the addition of Mn to Rh-based catalysts enhanced CO adsorption and found CO conversion was related with CO adsorption type and intensity. Mo et al.  found Fe decreased CO adsorption but improved hydrogenation ability of Rh-based catalysts. Liu et al.  found that the doping of Fe changed CO dissociation behavior over Rh/CeO2 catalyst during the experiments of CO adsorption and CO temperature programmed surface reaction (TPSR). Therefore, DRIFTS technique would be a very useful tool to understand the promotion of promoter from the view of elementary steps of ethanol formation, and to explain the difference in catalytic activity from a different view. Till now, the effect of Fe addition to Rh/Al2O3 on these steps has not been studied in detail. In this paper, DRIFTS experiments such as CO adsorption, CO hydrogenation, CO-TPSR were designated to explore the promotion effect of Fe on the catalytic performance of Rh/Al2O3 catalysts for C2 oxygenates synthesis via syngas.
Rhodium-based catalysts supported on γ-Al2O3 with different Fe loading were prepared by a co-impregnation method with aqueous solutions of iron nitrate [Fe(NO3)3], and rhodium nitrate [Rh(NO3)3]. After impregnation, the samples were dried at 110 °C for 12 h and calcined at 500 °C for 4 h. The obtained catalysts are designated as 2Rh-xFe/Al2O3, in which x ranges from 2 to 10 and the number before the metal element is weight percent relative to the mass of the γ-Al2O3 support.
H2-TPR as well as H2-TPD profile of the catalysts was recorded using a Micromeritics AutoChem II 2920 instrument. For H2-TPR experiments, the samples (0.2 g, 40–60 μm) were purged in argon stream at 500 °C for 30 min to remove traces of water and then cooled to room temperature. After that, the catalyst sample was exposed to 50 mL/min of 10% H2/Ar flow. Then, the sample was reduced from room temperature to 800 °C with an increase of 10 °C/min, and the TPR profile was recorded according to H2 consumption. As to H2-TPD measurements, the sample (0.2 g, 40–60 μm) was reduced in 50 mL/min of 10% H2/Ar flow at 350 °C for 2 h and was purged in He flow for another 30 min. After cooling to room temperature, H2 was introduced into the catalyst bed until saturation in a pulse mode. Subsequently, the catalyst bed was purged by He flow for 30 min and heated from room temperature to 800 °C under He flow at a ramp rate of 10 °C/min, while the desorbed products were detected with the TCD detector.
High-resolution scanning transmission electron microscopy (STEM) measurements were performed on a Tecnai G2 F30 S-TWIN electron microscope with 300 kV accelerating voltage via high-angle annular dark-field (HAADF). Point energy-dispersive X-ray spectroscopy (EDS) was taken in an area within 5 nm diameter.
DRIFTS experiments were carried out with a Nicolet 6700 spectrometer equipped with an MCT-A detector (Thermo, USA) in the range of 4000–650 cm−1. The DRIFTS experiments contain CO adsorption, CO temperature programmed surface reduction (CO-TPSR) and CO+H2. For the above experiments, sample pre-treatment was done as follows: the sample was put in the in situ cell and purged by pure nitrogen at 180 °C for 30 min. After that, the catalyst bed was ramped to 350 °C with a rise of 2 °C/min and reduced in flowing hydrogen for 2 h, and then, background spectra were collected at designated temperature under 10−4 mbar vacuum.
For CO adsorption, a flow of 5% CO/He (v/v) was introduced to the reduced catalyst for 30 min. The IR spectra were recorded after flushed by N2 for 30 min. For CO-TPSR, the IR spectra were recorded under 10% H2/Ar (v/v) flow with the temperate linearly increased from 30 to 260 °C after CO adsorption. For CO hydrogenation, the IR spectra were recorded under a mix gas flow of 5% CO/10%H2/85%He with the temperature linearly increased from 30 to 260 °C. All the spectra were recorded with 64 scans and a resolution of 4 cm−1.
Catalyst activity test
CO hydrogenation experiments were carried out in a fixed bed reactor at temperature of 260 °C, pressure of 2.0 MPa, space velocity of 3600 mL/(g. h) and H2/CO of 2:1. Prior to reaction, the catalysts (1 g, 40–60 mesh) were reduced in a H2 flow for 10 h at temperature of 350 °C and pressure of 0.1 MPa. Then, syngas was switched into the system. After steady state, the reaction was kept for 24 h to collect liquid samples. Outlet gases were online analyzed by two chromatographs (Agilent GC7890A). One was equipped with two thermal conductivity detector (TCD) to analyze CO, CO2, N2 and H2 using a 5-A molecular sieve column. The other one was one hydrogen flame ionization detector (FID) to analysis C1–C6 hydrocarbons using Plot Q column. The liquid products were offline analyzed with the chromatograph (Agilent GC7890A) fitted with one FID and one TCD using Plot Q column to separate C1–C6 liquid products and water.
Results and discussion
Temperature-programmed surface reduction (TPSR) is one of the most effective methods to study hydrogenation activity of chemically adsorbed CO [26, 27]. Methane is easily desorbed and its formation includes the steps as CO dissociation and hydrogenation, so its formation temperature and peak intensity is usually used as a tool to measure the CO dissociation ability and hydrogenation capacity of catalyst . Here, three Rh–Fe catalysts were selected to investigate the effect of Fe loading on hydrogenation behavior of adsorbed CO.
Catalytic performance of catalysts
Effect of Fe loading on performance of CO hydrogenation over Fe-promoted Rh-based catalysts a
Fe loading (%)
Product selectivity (C %)
From Table 1, it also can be seen that CO conversion increases with increasing Fe loading. The higher CO conversion of 2Rh-10Fe/Al2O3 should be assigned to the stronger hydrogenation capacity and the faster CO desorption rate according to CO-TPSR results. On the other hand, the selectivity of C2 oxygenates increases first and then decreases with the increase of Fe loading. The selectivity of C2 oxygenates present the highest with Fe proportion of 4 wt. %. Previous work suggests that Rh–Fe3+–O species are active sites for the generation of C2 oxygenates such as ethanol [5, 9]. Considering the contribution of Rh0 and Rh+ to CO insertion ability, the active site for ethanol formation over Rh-Fe/Al2O3 catalyst was assumed to the (Rhx0−Rhy+) -O-Fe3+ (Fe2+) sites with reference to that of Rh-Mn catalysts . These active sites could be formed by the close contact between Rh and Fe. From the results of TPR and STEM-EDS, we know that a part of Fe species was in close contact with Rh and a part of Fe existed as isolated Fe species. At the lower Fe loading, the catalyst has good synergistic effect between Rh and Fe and more Rh+ center which is mainly responsible for CO insertion. These increases Rh–Fe interface and provides more active sites for the formation of C2 oxygenates. However, the formation of C2 oxygenates should be a balance of CO dissociation, CO insertion and hydrogenation. 2Rh-2Fe/Al2O3 catalyst is more easily dissociated than 2Rh-4Fe/Al2O3 may be responsible for its slightly lower C2 oxygenate selectivity.
Promotion of Rh catalyst with Fe in the range of 2–10 wt. % was investigated. The addition of Fe facilitated the transfer of rhodium from bulk to surface and enhanced the hydrogen adsorption sites. The introduction of Fe inhibits CO adsorption especially bridge adsorption, correspondingly, the formation of total hydrocarbons decreases. CO adsorbed on the catalyst with lower Fe loading is more easily dissociated and CO adsorbed on the catalyst with higher Fe loading exhibited the strongest hydrogenation activity. The introduction of Fe also inhibits the desorption rate of CO (gem) and stabilizes CO (l) species.
Fe is an effective promoter to suppress the formation of hydrocarbons especially methane, and to shift selectivity to methanol and C2 oxygenates. However, it promotes water gas shift reaction, which leads to the increase in CO2 selectivity. The selectivity to C2 oxygenates in the products shows a rapid increase and then a slow decrease with the increase of Fe loading and passes through a maximum at 4wt. % of Fe loading.
This work was supported by the National Natural Science Foundation of China (31671797).
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